The nature of bone cell calcium regulation will be studied directly using an "in vitro" cell system. Cells will be isolated by collagenase digestion of 19-21 day old fetal rat calvaria. Ca-ATPase involvement in calcium efflux will be studied with bone cell membranes. A possible relationship of sodium and calcium transport will be investigated by studying the effect of various extracellular/intracellular ratios of sodium on 45Ca uptake and efflux. The effects of calcitonin transport will also be studied.